Question

In lac operon, repressor protein binds to

• A. Regulator gene
• B. Operator gene
• C. Promoter gene
• D. Structural gene

Answer 1

Answer: (B)

Operator gene

Regulator gene regulates the expression of structural genes by its protein products that are mostly transcription factors. These transcription factors/repressors bind to the operator region which is located just upstream the binding site of the RNA polymerase and hence prevent the binding and transcription. This makes option A incorrect and B correct. The operator is a DNA sequence present between the promoter region and the first coding gene. This operator region serves as repressor protein binding site; option B is correct. The promoter is a DNA segment that serves to initiate transcription of a particular gene. They are present upstream near the transcription start sites of genes and provide the binding site for RNA polymerase; option C is incorrect. The gene that codes for any RNA / protein product which does not serve as regulatory protein is known as structural genes. For example, lac z, lac y and lac a genes in lac operon are the structural genes; option D is incorrect. The correct option is B.